Among aromatic vinyl compounds, styrene especially is an industrially very important compound as a raw material for preparing polystyrene, synthetic rubber, ABS resin, etc. and has been industrially manufactured in a large amount.
In general, an aromatic vinyl compound is extremely susceptible to polymerization so that it easily undergoes polymerization under an influence of the heat generated in its preparation or purification stage, thereby causing problems of lowering the yield of the monomeric aromatic vinyl compound and, further, of forming soil (fouling) in the facilities involved to induce trouble in operating the facilities. As the countermeasure taken hitherto, a process wherein a certain polymerization inhibitor is added to process fluid containing an aromatic vinyl compound has been proposed and practically carried out. For example, there have been proposed a process employing phenols, nitrosophenols and/or nitrophenols (e.g. patent literature 1), one employing piperidine-1-oxyls (e.g. patent literature 2), one employing nitrophenols together with piperidine-1-oxyls) (e.g. patent literature 3), one employing an alkylsulfonic acid in combination with a dialkylhydroxylamine (e.g. patent literature 4), one employing a phosphoric acid ester and an alkylsulfonic acid in combination (e.g. patent literature 5), one adding phenol-, amine- and/or nitroso-related polymerization inhibitors together with dodecylbenzenesulfonic acid or its salt in order to suppress an increase in viscosity during an olefin preparation process (e.g. patent literature 6), and one adding a quinone-, hydroquinone- or nitroso-related polymerization inhibitor, or an amine-related polymerization inhibitor such as phenylenediamine together with dodecylbenzenesulfonic acid or its salt for preventing the formation of fouling in stage treating a vinyl compound (e.g. patent literature 7). Among them, especially 2,4-dinitrophenol (DNP) and 2,6-dinitro-4-methylphenol, 2,4-dinitro-6-sec-butylphenol (DNBP), which are nitrophenol compounds, have been often used as the polymerization inhibitors for the aromatic vinyl compound. However, these chemical agents show a tendency to gradually decrease their inhibitory power toward the polymerization reaction and necessitate a strict care in handling them because they belong to toxic substances. Accordingly, there has been strongly desired a process by which the effect of these chemical agents on polymerization-inhibition may be maintained and the amount used of them may be reduced.    Patent literature 1: JP 63-316745 A    Patent literature 2: JP 1-165534 A    Patent literature 3: JP 6-166636 A    Patent literature 4: U.S. Pat. No. 4,654,450 specification    Patent literature 5: U.S. Pat. No. 4,425,223 specification    Patent literature 6: JP 7-166152 A    Patent literature 7: JP 8-34748 A